Concomitant with increasing commercial use of fiber-optical communications, there is growing interest in optimized utilization of existing as well as of projected optical communications facilities. To a considerable extent, optimization of utilization depends on the quality of light sources.
For example, single optical fibers typically are used for transmission at frequencies at which there is appreciable frequency dispersion, laser sources are desired having high frequency stability or low "chirp" as pulses are transmitted. And, in the interest of close spacing of communications channels in coherent systems, lasers are desired having narrow linewidth.
One approach to making narrow-linewidth semiconductor lasers is suggested by R. C. Alferness et al., "Narrow-band Grating Resonator Filters in InGaAsP/InP Waveguides", Applied Physics Letters, Vol. 49 (1986), pp. 125-127. The suggested device includes a resonator filter comprising two diffraction grating sections exhibiting an effective phase shift relative to each other, device operation being understood in terms of a transmission resonance. As compared with the use of a (single) diffraction grating, a relatively short filter device in accordance with the reference achieves frequency selectivity corresponding to that of a much longer ordinary diffraction grating filter.
Further in the interest of simplified device manufacture, a laser design is desired which is readily suited for implementation in the form of hybrid assemblies.